The spread spectrum communication (hereinbelow abbreviated to SSC) is suitable for wireless data communication owing to its features such as the anti-disturbance, the secrecy, the toughness against multipath fading, etc.
According to the direct spread (hereinbelow abbreviated to DS) method among various SSC method, the transmitter transmits spread spectrum signals obtained by multiplying information signals to be transmitted with a PN code. The receiver is so constructed that received signals are correlated with the reference PN code provided with the receiver and that the correlation output is decoded so as to obtain information signals. Here, as means for correlating therebetween although there is known a correlating method using a mixer and a low path filter, or filter having a fixed weight, a convolver used as a correlator has a merit that the synchronization is effected with a high speed and that the PN code used therein is not fixed as for the matched filter, but it can be selected freely. A convolver used as a correlator in a receiver for SSC is disclosed, for example, in an article by J. H. Cafarella published in 1984 IEEE Ultrasonics Symposium Proceedings, pp. 121.about.26.
FIG. 7 shows the basic construction thereof, in which reference numerals 1 and 2 are gates; 3 is a convolver; 4 is a hybrid; IDT1 and IDT2 are input transducers, to which a received signal S and a reference signal R are supplied, respectively. This convolver 3 has a so-called double gate structure, in which two gates 1 and 2 are disposed on the propagation path of surface acoustic wave (hereinbelow abbreviated to SAW) so that two convolution outputs are obtained. The two convolution outputs are inputted to the hybrid 4, which is constructed so as to obtain an output O.sub.1 representing the sum of the two convolution outputs and another output O.sub.2 representing the difference therebetween. This construction is called a DPSK convolver. DPSK means the differential phase shift keying and according to the DPSK on the transmitter side, an information signals is DPSK-encoded and a spread-spectrum (hereinbelow abbreviated to SS)-encoded signal on the receiver side, is correlated with the reference signal by means of a DPSK convolver and thus it can be DPSK-decoded.
A parameter called BT product is used as an index representing characteristics of the convolver. B indicates the band width of the convolver and T is a delay time corresponding to the gate length. The BT product is a value obtained by multiplying these two values and the greater it is, the better it is. B is limited principally by the design of the two transducers IDT1 and IDT2 and the material of the SAW element. In order to make the BT product greater, T may be enlarged. However, when T is too large, the length of the SAW element is too great and further the longer it is, the more difficult the fabrication is and the higher the cost is. In FIG. 7, if L is restricted, the length of the gate 1 and that of the gate 2 are about L/2, respectively.
Therefore, the construction of a convolver, as indicated in FIG. 8, may be conceivable. In FIG. 8, two convolvers, each of which has a gate length of L, are used and each of the outputs thereof are inputted to the hybrid. In this way, it is possible to obtain a BT product equivalent to that of a convolver, whose gate length is twice as long as that indicated in FIG. 7.
However, there is a problem in this construction of the convolver. FIG. 9 indicates the ideal cooperating mode of the convolver having the construction indicated in FIG. 8. S1.about.S4 show correlation spike waveforms of the outputs O'.sub.1 and O'.sub.2 of the two convolvers. S1, S2 and S3 have a same amplitude and a same phase. However, S4 has the same amplitude, but its phase is inversed. S5 and S6 are outputs of the hybrid 4 and the following relations are valid: EQU S5=S1+S3 (1) EQU S6=S2-S4 (2)
at the output O.sub.1, since EQU S2+S4=0 (3)
for the region indicated by B there appears no output. On the other hand, at the output O.sub.2, since EQU S1-S3=0 (4)
there appears no output for the region indicated by A.
The output O.sub.1 represents data of "1", while the output O.sub.2 represents data of "0". In this case, if the characteristics of the convolver 4.sub.1 are not identical to those of the convolver 4.sub.2 and the amplitude and the phase of the outputs are not as indicated in FIG. 9, deterioration phenomena that the amplitude of S5 of the output O.sub.1 or that of S6 of the output O.sub.2 may be reduced, or a correlation spike may appear at the region indicated by B for the output O.sub.1 or at the region indicated by A for the output O.sub.2 are produced, which makes the judgment difficult. Further, even if the amplitude and the phase are adjusted, when variations in characteristics of the convolver due to temperature variations are taken into account, the adjustment is not easy.